Abstract Detail

Leaf-Branch Complex: the Common Bau-Plan for Reproductive Organs of Land Plants.

Previously the hypothesis advanced by Arber and Parkin (1907) stated that the ancestral angiosperms were Magnolia-like, and the ancestral conduplicate carpels were derived from former megasporophylls bearing ovules along their margins. This idealized speculation, although dominating and of great influence in textbooks, was apparently ruthlessly not favored by plants and their fossils, leaving the origin of angiosperms a perpetual mystery and headache for many. However, increasing evidence (including morphology, anatomy, function genes, and fossils) seems to favor the Unifying Theory, which states that the angiosperm gynoecium (including carpels) is composed of leaves and ovuliferous branches, or putting in another way, the gynoecium in angiosperms is an aggregation of leaves and ovuliferous branches. This interpretation, although conflicting with former dominating hypotheses, makes reproductive organs of angiosperms more comparable with those of gymnosperms, as the latters are also proven composed of bracts (foliar parts) and ovuliferous branches. If accepted, this interpretation paves the road to correlating seed plants with earliest land plants, Cooksonia, in which all sporangia are borne on the termini of branches and have no leaves (which can be taken as branches sterilized and modified in later evolution). After this integration, the reproductive organs of ferns and ferns-like plants also become comprehensible: the so-called fertile pinnae are actually aggregations of foliar parts (two-dimensionalized sterilized branches) and branches bearing sporangia on their termini. Compared with former textbook hypotheses on evolution of plant reproductive organs, this new hypothesis seems to be comprehensive, as it reveals the common Bau-plan shared by the reproductive organs of all land plants. This hypothesis, although plausible macroscopically, needs to be further tested by molecular studies and calls for integration between currently divorced plant morphology and molecular studies.